Ultra-narrow line width Tm³⁺-doped fiber laser based on intra-core fiber Bragg gratings

A 74 pm ultra-narrow linewidth diode laser (LD) claddingding-pumped Tm³⁺-doped fiber laser was reported. The maximal continuous wave (CW) output power was 14.5 W at 1946 nm, with a slope efficiency of 37.5% and the threshold of 5.3 W. The single-mode double-cladding Tm³⁺-doped fiber had a core diameter of 15 μm (NA:0.1), and the regular hexag-onal inner-cladding diameter was 200 μm (NA:0.46). The high reflection coupler was based on the intra-core fiber Bragg grat-ings (FBGs), which were directly written into the Tm³⁺-doped non-photosensitive fiber core by the 800 nm femtosecond laser and a phase mask. The output coupler was the other fiber end with 3.6% Fresnel reflection, and the pump source was a high power 792 nm LD with a 200 μm pigtail fiber output. According to our knowledge, this was the first report on the ultra-narrow linewidth Tm³⁺-doped fiber laser using intracore FBGs approximately 2 μm. The beam radius for the fiber laser at the 14 W output power level. Solid curve was a fit to a standard Gaussian beam propagation expression.